The accurate measurement of temperature is an important aspect of a metal rolling process. The surface temperature of the metal as it is being rolled, and the surface temperature of the rollers, are important parameters in the rolling process. For some metals in particular, such as aluminum, maintaining the surface temperature within a well-defined range is a very important requirement if high quality aluminum is to be produced. If the desired temperature control is not met, the quality of the resulting aluminum will be lower.
Controlling the temperature assumes however that the temperature can be reliably measured, and in the context of an aluminum rolling process this is difficult to achieve. The ambient conditions, including a very damp atmosphere owing to the presence of large amounts of lubricating fluid, provide an environment in which the accurate and reliable measurement of temperature is difficult to achieve.
A number of known techniques have been used in an attempt to measure surface temperatures in processes such as metal rolling with varying degrees of success.
Infrared pyrometry techniques are well-known and popular methods for measuring surface temperature and have the advantage of being non-contact. They do however have a number of disadvantages, especially in the context of measuring the surface temperature of a metal such as aluminum during a rolling process. Firstly, for example, pyrometric techniques depend on the emissivity of the surface. This is a particular problem for measuring the surface temperature of a metal such as aluminum which is reflective and has a low emissivity. Pyrometry can also be disturbed by the presence of particles in the light path, such as dirt, dust and water vapor. These problems can be particularly severe in an aluminum rolling mill. Attempts to solve this problem include the use of multi-wavelength radiation detectors with an air stream to minimize clogging of the lens system by dust particles, and by using a fiber optic cable to bring the optical system closer to the surface. However, when the target surface is covered with lubricants, even multi-wavelength radiation detectors cannot measure the true surface temperature because they measure the temperature of the lubricants rather than the metal surface. Blowing the surface with an airstream to clear the lubricants will artificially cool the surface. Furthermore these solutions increase further the cost of what is already an expensive measurement technique.
A further disadvantage to pyrometric techniques is that the measurement error increases with increasing wavelength of the radiation used for the measurement. Ideally therefore the shortest possible wavelength should be chosen. However, at comparatively low temperatures (e.g., below about 300° C.) the intensity of the radiation becomes very low or even close to zero at short wavelengths. Thus pyrometry is a technique best suited to relatively high temperatures, but in an aluminum rolling process it is often necessary to control temperatures that are below 300° C.
While pyrometry has the advantage of being a non-contact measurement technique, small gap measurement techniques using thermocouples have also been used. These methods involve the use of a thermocouple positioned in very close proximity to the surface. Commercial instruments, such as, the CO 15 series thermocouple provided by CHINO of Tokyo, Japan, are available and such instruments are designed for the measurement of the surface temperature of moving objects such as rollers and moving sheets. The thermocouple has a built-in automatic correction circuit for the difference between the surrounding air and actual temperature of an object. No emissivity compensation to the object surface is necessary. The measurement ranges from 1 mm to 6 mm depending on the model.
However, temperature measurement using the small gap method depends critically on maintaining a constant air gap between the sensor and the surface. Thermal expansion, eccentricities of the rollers and other variables often affect the width of the gap and culminate in significant errors. Moreover, the result is also seriously affected by the presence of lubricant film, water vapor and turbulent air, all of which may be common in aluminum rolling mills.
Also known in the art are numerous methods that require a temperature measuring probe to contact the surface the temperature of which is to be measured. The most common type consists of a thermocouple strip with the hot junction located at the middle of the strip. The strip is designed to be pressed onto the target surface for thermal contact. Pressure on the strip is limited by a polytetraflouroethylene (PTFE) (for example, DuPont's TEFLON® brand polytetraflouroethylene) or stainless steel rollers or guides attached to the measuring head of the probe. Another known method, for example, includes a spring loaded thermocouple tip welded onto a copper disc for thermal contact.
Commercially available contact surface probes usually have measurement errors of 5 to 20° C. depending on factors such as local surface flatness and the extent of matching between the thermal contact surfaces. Contact measurement on stationary surfaces always give errors that are lower than the true value because of the temperature drop across the contact interface. The error limits become wider on moving surfaces due to factors such as frictional heat and variations in surface contact conditions etc.
Another disadvantage of such prior art devices is that commercial surface contact probes tend to produce a mark or groove on soft metal surfaces such as aluminum. Dirt and other particles may also accumulate at the contact probe resulting in unsightly marks on the metal surface. On rollers these probes also tend to disturb the loosely adhered aluminum oxide on the surface, which could have undesirable effects on the appearance of the sheet product.